1. Field of the Invention
The present invention relates generally to the field of automatic level control for radio frequency (RF) power amplifiers and, more specifically, to automatic level control for a transmitter in a cellular telephone transceiver.
2. Discussion of the Prior Art
Cellular telephone systems presently in service in the United States and many other countries operate using analog, frequency modulated (FM) carrier signals. In accordance with prevailing operating protocols, two different carrier frequencies are used (one each for transmission and reception) for each call in which one of the parties is using a cellular transceiver. During the course of a call, analog transceivers transmit continuously using one of several discrete power levels which range from a few milliwatts to several watts. A transceiver's power level is dynamically assigned by the base station processing the call. A base station typically adjusts a given transceiver's power level in response to changes in the strength of the signal received by the base station from that transceiver. Thus, a transceiver which is relatively far away from the base station may be directed to transmit using the highest power level, while another transceiver in close proximity to the base station may be instructed to use a lower power level.
The prevailing protocol in the U.S. requires that the actual output power level of the transceiver remain within a narrow dynamic range around the assigned power level at all times and over a wide ambient temperature range. Such precise control is necessary to minimize interference between transceivers operating in close proximity.
In a proposed next generation of cellular telephone service, referred to as the EIA IS-54 Dual Mode Cellular System, communications are carried out in a significantly different fashion. First, in order to increase capacity through more efficient use of the electromagnetic spectrum allocated for cellular service, a time division multiple access (TDMA) protocol is proposed in which each communication channel (carrier frequency) is time divided into a cyclical series of time slots or "frames." Multiple transceivers share a single communication channel by transmitting information in digital form only during the frames assigned to them (burst transmission). During transmission, however, each transceiver uses a discrete output power level assigned by the base station, just as in current analog systems. In this manner, multiple simultaneous calls are carried by a single frequency, as opposed to one transmit carrier frequency per call as required by analog systems.
Since each transceiver may transmit only during designated frames (and not during other times), but must still adhere to the power level assigned by the base station, conventional output power control circuitry is generally incapable of satisfactory performance. For example, it is well known to use a negative feedback loop to control the output power level of a cellular telephone's transmitter. This is typically accomplished by providing suitable circuitry to sample the transmitter's output signal, to rectify the sample and then compare the rectified signal with a reference which is representative of the "correct" or desired power level. The difference or error signal yielded by the comparison is used to adjust the amplifier's output power level.
For this type of conventional feedback circuit to function, however, the transmitter or amplifier must operate continuously and the feedback loop must remained closed. That is, if the amplifier is turned off or the feedback loop is opened, control of the output power level is at least temporarily lost. Moreover, the loop response time of a conventional feedback circuit is typically too long (slow) in comparison with the duration of a frame in the proposed TDMA system. An excessively slow response time prevents the loop from stabilizing the output power level at the appropriate level before the frame has ended and the amplifier is no longer transmitting.
Similarly, the loop response time of a conventional feedback circuit is typically too short (fast) in comparison with the amplitude component of the digital modulation used in the proposed TDMA system. An adverse effect of the excessively short loop response time is that as the output power level varies in accordance with the amplitude modulation, the conventional feedback loop will tend to eliminate such variations and thus effectively eliminate the desired modulation. Thus, under the proposed TDMA system in which each transceiver transmits in "bursts" corresponding to its assigned frames, such conventional feedback circuits would not be capable of controlling the output power level as required by the protocol.
In addition, since base stations supporting the proposed TDMA protocol will be introduced gradually in most service areas and will not immediately supplant existing analog service in such areas, this transition gives rise to a compatibility problem. This problem is caused by the fact that some geographical areas will be served only by analog service while others will be served by TDMA service. Thus, as provided in the EIA IS-54 interim specification, next generation transceivers must be capable of operating with either type of service.